The field of the invention is that of writable data media such as optical disks and especially data media on which the writing of data is distributed by sectors.
Under the invention, each sector is referenced by a binary word pre-recorded onto the data medium. Thus, to access a sector, a read or write head scans the data medium until it detects this binary word on it.
Generally, a blank optical disk is not completely blank. A recording track is prestamped onto the disk. Often, this track takes the form of a spiral groove whose depth is equal to a quarter of a wavelength of the laser ray emitted by a read head. During writing to the disk, the read head follows the groove so as to keep a writing laser beam inside, next to or alternatively inside and outside of the groove.
The groove is in the form of a spiral on a macroscopic scale and sinusoidal (referred to as wobble) on a microscopic scale. The sinusoidal form is mainly used to measure the linear speed of the disk passing under the read head so as to control this speed.
According to a first known prior art, a succession of pre-positioned pits (referred to as prepits) locally in each sector, forms the binary word that references this sector. These pits are pre-positioned inside or next to the groove so as to be able to identify an absolute position of the sector by means of the read head when it follows the groove.
The frequency at which the succession of pre-positioned pits passes under the read head makes this coding scheme particularly sensitive to high-frequency noise. This sensitivity to noise causes errors in decoding the succession of pre-positioned pits to obtain the binary word that references the sector.
Another solution consists in coding the binary word by modifying certain alternations of the groove wobble. For example, a modified alternation may represent a first binary value and, vice versa, a preserved alternation may represent a second binary value complementary to the first. The alternation modifications must be made so as not to interfere with the detection of the original alternation by the read head in its feedback control functions for following the groove and calculating the speed of travel of the disk under the head.
Writing data, such as NRZ (Non-Return to Zero) data, to the disk is normally done by modulating the power of the write head laser beam in the vicinity of the read head. When the signal resulting from alternation modifications is read while writing data to the disk, the signal read on the disk is interfered with by the writing laser beam. This is the cause of errors in decoding alternation modifications for the binary word referencing the sector on which the data to be recorded is intended to be written.
In order to write the data to the proper sector as specified, errors in decoding for obtaining the binary word referencing the sector should be overcome.